The present invention relates to a mechanism whose role is to orientate, position or move a useful load sensitive to the orientation, in a precise specific direction or to position or to transmit motion to a useful load in a specific way with respect to a fixed platform.
The different functions fulfilled by such a mechanism, which will be described further on, whether their purpose is to position a useful load in a given angular position or to synchronize the movement, in consequence of the initial angular movement because the device is supported on a structure, or whether their purpose is to transmit motion to the useful load in opposition to a resistance or inertia or to supply a force directed against a second device, all these functions will be named in what follows "orientation and positioning of the useful load".
It is generally required of mechanisms which ensure the orientation and the positioning of a useful load, that they present a certain number of characteristics. Among these characteristics there is included, for example, the ability to support the device before, during and after orientation and positioning of the useful load, the ability to provide an orientation and positioning method which has the required accuracy either because of their own performance or because of their ability to respond to orders coming from an external control means, the characteristics represented by a small mass and a small volume, their ease of fixing to a mounting point and to the useful load, their reliability and the face that the break-downs to which these mechanisms may be subjected must nevertheless allow the useful load to maintain favorable orientation and positioning, as well as other characteristics required by the specific application.
Such mechanisms which provide orientation and positioning of the useful load, present applications which extend from aiming large telescopes to the positioning of delicate instruments. Such applications may comprise, for example, gyroscope and flight instrument tables in aircraft, orientation supports for telescopes, vertical reference platforms, elbow or cranked joints for articulated apparatus or robot devices and numerous other applications. In satellites, positioning and orientation devices for useful loads are needed to fulfill numerous functions, for example aiming and orientating solar panels, aiming masts and antennae, the orientation of inertial moment wheels, the aiming of instruments, the orientation of thrust devices and other exploration and positioning services.
The application of the basic principle of the present invention to these different functions may be easily understood with reference to the example given and to the corresponding descriptions. One example of an application in which the advantages of the invention are evident is that of antennae aiming in satellites. Such devices are commonly called "antenna pointing mechanism" and the abbreviation "APM" will be used to designate them in what follows.
The state of the art comprises antenna pointing devices of different generic types whose general diagram shown in FIGS. 1 and 2 illustrate two widely used systems.
FIG. 1 is a schematic representation of an antenna pointing device of the gimbal type. The essential structural parts of this device are the basic structure 1, the gimbal structure 2 and the useful load structure. The gimbal structure 2 is connected to the base structure 1 by means of a bearing or pivot suspension (not shown), and it is driven by an actuating device which may be in the form of a direct electromagnetic control or connected through a gear box, between the two structural parts, or by means of a linear actuating device applied between the two structures so as to define a relative rotation about axis A. The antenna structure is mounted on the gimbal structure by means of which it is driven, which allows it to acquire its position along two axes with respect to the interface of the satellite.
FIG. 2 is a schematical representation of an antenna pointing mechanism of the universal joint type in which the antenna is supported on a cross-shaped structure 4, itself mounted on a universal joint 5, that is to say a suspension having two or more axes with central disposition. Such a joint could be formed specifically by a ball and socket joint or by a twin axis gimbal bearing system or by a flexible element. In the application of this type of antenna pointing mechanism, the basic structure may be omitted and the surface of the satellite may be used and, similarly, the antenna may be mounted directly on the universal joint and be actuated directly. However, the fact of omitting these structures precludes control of the APM as a single mechanism and complicates its use.
These devices of the prior art present some drawbacks which are more especially the following:
in so far as their load capacity is concerned, the ability of these devices to provide a low mass with accurate bearing mounting and to support, at the same time, the load through the launching environment of the satellite, is very limited and it is consequently frequently necessary to include in the mechanism a so-called "safety" device which protects the APM during launching and which is freed by a separate actuating device such as a pyrotechnic system as soon as the satellite has reached its operational orbit;
in so far as the accuracy of pointing is concerned, these mechanisms have varying performances depending on their operating mode. When the positional information comes from inside the APM itself, their size and open form of construction make them sensitive to thermal variations, temperature gradients through the mechanism being particularly destructive from the point of view of obtaining high pointing accuracy. In the closed loop mode of construction which uses an external error sensor, high accuracy may be obtained in generally all types of APM, as long as a sufficiently small angular differential increase may be imposed on each axis independently and as long as the backlash is small and the rigidity high and as long as the control system is able to respond to errors in time;
in so far as their mass in concerned, these devices of the prior art normally have a high mass due to their very construction and to the mass of the systems which are associated therewith-pyrotechnic systems, safety devices and additional structures required for the support;
in so far as the ease is concerned with which these systems may be fixed to the satellite or to the useful load, the large size of these devices limits use thereof, more particularly that of the universal joint device, to where there is a large mounting area;
in so far as the breakdowns to which they may be subjected is concerned, it is often desirable for an APM system, which normally maintains the antenna pointed in a fixed direction but which may be required to periodically re-point the antenna under control in a different direction, to be able to come back to its nominal fixed direction in the case of breakdown of one of the actuating devices which control the axes. This is extremely difficult to obtain and the problem is usually resolved by adopting pyrotechnic triggering devices which uncouple the actuating device affected (which is not always necessary if, for example, a motor drives the axis directly without going through a gear system, and by providing the APM with another device for bringing the antenna back to its nominal position and another pyrotechnic system for locking the antenna safely in this position;
in so far as the complexity is concerned, it is soon evident that the devices of the prior art use a greater complexity for achieving their function than a device such as the one forming the subject of the present invention, which will be described in what follows.